java software solutions lewis and loftus chapter 5 1 copyright 1997 by john lewis and william...

Java Software Solutions Lewis and Loftus

More Programming Constructs -- Introduction

• We can now examine some additional programming concepts and constructs

• Chapter 5 focuses on:

– internal data representation

– conversions between one data type and another

– more operators

– more selection statements

– more repetition statements

Internal Data Representation

• We discussed earlier that every piece of information stored on a computer is represented as binary values

• What is represented by the following binary string?

01100001001010

• You can't tell just from the bit string itself.

• We take specific binary values and apply an interpretation to them

Representing Integers

• There are four types of integers in Java, each providing a different bits to store the value

• Each has a sign bit. If it is 1, the number is negative; if it is 0, the number is positive

s 7 bits

s 15 bits

s 31 bits

s 63 bits

Two's Complement

• Integers are stored in signed two's complement format

• A positive value is a straightforward binary number

• A negative value is represented by inverting all of the bits of the corresponding positive value, then adding 1

• To "decode" a negative value, invert all of the bits and add 1

• Using two's complement makes internal arithmetic processing easier

Two's Complement

• The number 25 is represented in 8 bits (byte) as

00011001

• To represent -25, first invert all of the bits

11100110

then add 1

11100111

• Note that the sign bit reversed, indicating the number is negative

Overflow and Underflow

• Storing numeric values in a fixed storage size can lead to overflow and underflow problems

• Overflow occurs when a number grows too large to fit in its allocated space

• Underflow occurs when a number shrinks too small to fit in its allocated space

• See Overflow.java

Representing Floating Point Values

• A decimal (base 10) floating point value can be defined by the following equation

sign * mantissa * 10 exponent

• where

– sign is either 1 or -1

– mantissa is a positive value that represents the significant digits of the number

– exponent is a value that indicates how the decimal point is shifted relative to the mantissa

Representing Floating Point Values

• For example, the number -843.977 can be represented by

-1 * 843977 * 10 -3

• Floating point numbers can be represented in binary the same way, except that the mantissa is a binary number and the base is 2 instead of 10

sign * mantissa * 2 exponent

• Floating point values are stored by storing each of these components in the space allotted

Representing Characters

• As described earlier, characters are represented according to the Unicode Character Set

• The character set matches a unique number to each character to be represented

• Storing the character is therefore as simple as storing the binary version of the number that represents it

• For example, the character 'z' has the Unicode value 122, which is represented in 16 bits as

0000000001111010

Representing Characters

• Because they are stored as numbers, Java lets you perform some arithmetic processing on characters

• For example, because 'A' is stored as Unicode value 65, the statement

char ch = 'A' + 5;

will store the character 'F' in ch (Unicode value 70)

• This relationship is occasionally helpful

Conversions

• Each data value and variable is associated with a particular data type

• It is sometimes necessary to convert a value of one data type to another

• Not all conversions are possible. For example, boolean values cannot be converted to any other type and vice versa

• Even if a conversion is possible, we need to be careful that information is not lost in the process

Widening Conversions

• Widening conversions are generally safe because they go from a smaller data space to a larger one

• The widening conversions are:

byteshortcharintlongfloat

short, int, long, float, or doubleint, long, float, or doubleint, long, float, or doublelong, float, or doublefloat or doubledouble

Narrowing Conversions

• Narrowing conversions are more dangerous because they usually go from a smaller data space to a larger one

• The narrowing conversions are:

byteshortcharintlongfloatdouble

charbyte or charbyte or shortbyte, short, or charbyte, short, char, or intbyte, short, char, int or longbyte, short, char, int, long, or float

Performing Conversions

• In Java, conversion between one data type and another can occur three ways

• Assignment conversion - when a value of one type is assigned to a variable of another type

• Arithmetic promotion - occurs automatically when operators modify the types of their operands

• Casting - an operator that forces a value to another type

Assignment Conversion

• For example, if money is a float variable and dollars is an int variable (storing 82), then

money = dollars;

converts the value 82 to 82.0 when it is stored

• The value in dollars is not actually changed

• Only widening conversions are permitted through assignment

• Assignment conversion can also take place when passing parameters (which is a form of assignment)

Arithmetic Promotion

• Certain operators require consistent types for their operands

• For example, if sum is a float variable and count is an int variable, then the statement

result = sum / count;

internally converts the value in count to a float then performs the division, producing a floating point result

• The value in count is not changed

Casting

• A cast is an operator that is specified by a type name in parentheses

• It is placed in front of the value to be converted

• The following example truncates the fractional part of the floating point value in money and stores the integer portion in dollars

dollars = (int) money;

• The value in money is not changed

• If a conversion is possible, it can be done through a cast

More Operators

• We've seen several operators of various types: arithmetic, equality, relational

• There are many more in Java to make use of:

– increment and decrement operators

– logical operators

– assignement operators

– the conditional operator

The Increment and Decrement Operators

• The increment operator (++) adds one to its integer or floating point operand

• The decrement operator (--) subtracts one

• The statement

count++;

is essentially equivalent to

count = count + 1;

• The increment and decrement operators can be applied in prefix (before the variable) or postfix (after the variable) form

• When used alone in a statement, the prefix and postfix forms are basically equivalent. That is,

count++;

is equivalent to

++count;

• When used in a larger expression, the prefix and postfix forms have a different effect

• In both cases the variable is incremented (decremented)

• But the value used in the larger expression depends on the form

Expression

count++++countcount----count

Operation

add 1add 1

subtract 1subtract 1

Value of Expression

old valuenew valueold valuenew value

• If count currently contains 45, then

total = count++;

assigns 45 to total and 46 to count

• If count currently contains 45, then

total = ++count;

assigns the value 46 to both total and count

• If sum contains 25, then the statement

System.out.println (sum++ + " " + ++sum +

" " + sum + " " + sum--);

prints the following result:

25 27 27 27

and sum contains 26 after the line is complete

Logical Operators

• There are three logical operators in Java:

• They all take boolean operands and produce boolean results

• Logical NOT is unary (one operand), but logical AND and OR are binary (two operands)

Operator

Operation

Logical NOTLogical ANDLogical OR

Logical NOT

• The logical NOT is also called logical negation or logical complement

• If a is true, !a is false; if a is false, then !a is true

• Logical expressions can be shown using truth tables

falsetrue

truefalse

Logical AND

• The expression a && b is true if both a and b are true, and false otherwise

• Truth tables show all possible combinations of all terms

falsefalsetruetrue

falsetruefalsetrue

a && b

falsefalsefalsetrue

Logical OR

• The expression a || b is true if a or b or both are true, and false otherwise

falsefalsetruetrue

falsetruefalsetrue

a || b

falsetruetruetrue

Logical Operators

• Conditions in selection statements and loops can use logical operators to form more complex expressions

if (total < MAX && !found)

System.out.println ("Processing...");

• Logical operators have precedence relationships between themselves and other operators

Logical Operators

• Full expressions can be evaluated using truth tables

total < MAX

falsefalsetruetrue

falsetruefalsetrue

!found

truefalsetruefalse

total < MAX && !found

falsefalsetruefalse

Assignment Operators

• Often we perform an operation on a variable, then store the result back into that variable

• Java provides assignment operators that simplify that process

• For example, the statement

num += count;

is equivalent to

num = num + count;

• There are many such assignment operators, always written as op= , such as:

Operator

+=-=*=/=%=

Example

x += yx -= yx *= yx /= yx %= y

Equivalent To

x = x + yx = x - yx = x * yx = x / yx = x % y

• The right hand side of an assignment operator can be a complete expression

• The entire right-hand expression is evaluated first, then combined with the additional operation

• Therefore

result /= (total-MIN) % num;

is equivalent to

result = result / ((total-MIN) % num);

The Conditional Operator

• Java has a conditional operator that evaluates a boolean condition that determines which of two expressions is evaluated

• The result of the chosen expression is the result of the entire conditional operator

• Its syntax is:

condition ? expression1 : expression2

• If the condition is true, expression1 is evaluated; if it is false, expression2 is evaluated

• It is similar to an if-else statement, except that it is an expression that returns a value

• For example:

larger = (num1 > num2) ? num1 : num2;

• If num1 is greater that num2, then num1 is assigned to larger; otherwise, num2 is assigned to larger

• The conditional operator is ternary, meaning it requires three operands

• Another example:

System.out.println ("Your change is " + count +

(count == 1) ? "Dime" : "Dimes");

• If count equals 1, "Dime" is printed, otherwise "Dimes" is printed

Another Selection Statement

• The if and the if-else statements are selection statements, allowing us to select which statement to perform next based on some boolean condition

• Another selection construct, called the switch statement, provides another way to choose the next action

• The switch statement evaluates an expression, then attempts to match the result to one of a series of values

• Execution transfers to statement list associated with the first value that matches

The switch Statement

• The syntax of the switch statement is:

switch (expression) {

case value1:

statement-list1

case value2:

statement-list2

case …

The switch Statement

• The expression must evaluate to an integral value, such as an integer or character

• The break statement is usually used to terminate the statement list of each case, which causes control to jump to the end of the switch statement and continue

• A default case can be added to the end of the list of cases, and will execute if no other case matches

• See Vowels.java

More Repetition Constructs

• In addition to while loops, Java has two other constructs used to perform repetition:

• the do statement

• the for statement

• Each loop type has its own unique characteristics

• You must choose which loop type to use in each situation

The do Statement

• The do statement has the following syntax:

statement

while (condition);

• The statement is executed until the condition becomes false

• It is similar to a while statement, except that its termination condition is evaluated after the loop body

The do Statement

statement

condition

The do Statement

• See Dice.java

• The key difference between a do loop and a while loop is that the body of the do loop will execute at least once

• If the condition of a while loop is false initially, the body of the loop is never executed

• Another way to put this is that a while loop will execute zero or more times and a do loop will execute one or more times

The for Statement

• The syntax of the for loop is

for (intialization; condition; increment)

statement;

which is equivalent to

initialization;

while (condition) {

statement;

increment;

The for Statement

• Like a while loop, the condition of a for statement is tested prior to executing the loop body

• Therefore, a for loop will execute zero or more times

• It is well suited for executing a specific number of times, known in advance

• Note that the initialization portion is only performed once, but the increment portion is executed after each iteration

The for Statement

statement

conditionfalse

initialization

increment

The for Statement

• Examples:

for (int count=1; count < 75; count++)

System.out.println (count);

for (int num=5; num <= total; num *= 2) {

sum += num;

System.out.println (sum);

• See Dice2.java

The for Statement

• Each expression in the header of a for loop is optional

– If the initialization is left out, no initialization is performed

– If the condition is left out, it is always considered to be true, and therefore makes an infinite loop

– If the increment is left out, no increment opertion is performed

• Both semi-colons are always required

The break and continue statements

• The break statement, which we used with switch statements, can also be used inside a loop

• When the break statement is executed, control jumps to the statement after the loop (the condition is not evaluated again)

• A similar construct, the continue statement, can also be executed in a loop

• When the continue statement is executed, control jumps to the end of the loop and the condition is evaluated

The break and continue Statements

• They can calso be used to jump to a line in your program with a particular label

• Jumping from one point in the program to another in an unstructured manner is not good practice

• Therefore, as a rule of thumb, avoid the break statement except when needed in switch statements, and avoid the continue statement altogether

java software solutions lewis and loftus chapter 5 1 copyright 1997 by john lewis and william...

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